Chlorine dioxide (ClO2) has many industrial and municipal uses. When produced and handled properly, ClO2 is an effective and powerful biocide, disinfectant and oxidizer.
ClO2 is also used extensively in the pulp and paper industry as a bleaching agent, but is gaining further support in such areas as disinfections in municipal water treatment. Other end-uses can include as a disinfectant in the food and beverage industries, wastewater treatment, industrial water treatment, cleaning and disinfections of medical wastes, textile bleaching, odor control for the rendering industry, circuit board cleansing in the electronics industry, and uses in the oil and gas industry.
In water treatment applications, ClO2 is primarily used as a disinfectant for surface waters with odor and taste problems. It is an effective biocide at low concentrations and over a wide pH range. ClO2 is desirable because when it reacts with an organism in water, chlorite results, which studies to date have shown does not pose a significant adverse risk to human health at a concentration of less than 0.8 parts per million (ppm) of chlorite. The use of chlorine, on the other hand, can result in the creation of chlorinated organic compounds when treating water. Such chlorinated organic compounds are suspected to increase cancer risk.
Producing ClO2 gas for use in a ClO2 water treatment process is desirable because there is greater assurance of ClO2 purity when in the gas phase. ClO2 is, however, unstable in the gas phase and will readily undergo decomposition into chlorine gas (Cl2), oxygen gas (O2), and heat. The high reactivity of ClO2 generally requires that it be produced and used at the same location. ClO2 is, however, soluble and stable in an aqueous solution.
The production of ClO2 can be accomplished both by electrochemical and reactor-based chemical methods. Electrochemical methods have an advantage of relatively safer operation compared to reactor-based chemical methods. In this regard, electrochemical methods employ only one precursor, such as a chlorite solution, unlike the multiple precursors that are employed in reactor-based chemical methods. Moreover, in reactor-based chemical methods, the use of concentrated acids and chlorine gas poses a safety concern.
Electrochemical cells are capable of carrying out selective oxidation reaction of chlorite to ClO2. The selective oxidation reaction product is a solution containing ClO2. To further purify the ClO2 gas stream, the gas stream is separated from the solution using a stripper column. In the stripper column, air is passed from the bottom of the column to the top while the ClO2 solution travels from top to the bottom. Pure ClO2 is exchanged from solution to the air. Suction of air is usually accomplished using an eductor, as described in copending and co-owned application Ser. No. 10/902,681. However, a vacuum gas transfer pump can alternatively be employed.
An electrochemical ClO2 generator, such as those described and claimed in the '681 and '398 applications, can be utilized to obtain a higher yield of ClO2 gas or ClO2 solution than those previously disclosed. This can be accomplished by applying a higher current to the electrochemical cell than those previously applied. Applying a higher current to the cell increases the rate of the selective oxidation reaction of chlorite to ClO2, which results in a higher yield of ClO2 gas. A higher yield of ClO2 gas ultimately results in a higher yield of ClO2 solution.
However, the electrolytic cells described in the '681 and '398 applications cannot be safely operated at these higher currents. It is known that ClO2 is unstable and capable of decomposing, in an exothermic reaction, to chlorine and oxygen. Due to this instability, an operating temperature greater than about 163° F. (73° C.) can result in potentially hazardous and less efficient operation of the ClO2 generator.
When more current is applied to the electrochemical cell, more heat is generated in the electrolytic cell anolyte loop. This is problematic because the temperature increase of the electrolytic cell anolyte loop can create an unsafe chlorine dioxide temperature in the anolyte loop such as within the stripper column or at other location in the ClO2 generator such as at the gas transfer pump.
Accordingly, it would be desirable to provide a ClO2 generator capable of operating at a high current. Moreover, it would be desirable that the ClO2 generator have temperature control mechanisms within the chlorine dioxide gas source or anolyte loop such as in the stripper column, around the feed lines to or from the electrochemical cell, or around or within the inlet/outlet pipes to the gas transfer pump.